HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 179

contaminant. Contaminant-free air is injected into the soil through a vertical
well using a blower in order to implement volatilization, which is the process
of converting a solid or liquid into the vapor phase. An extraction well is put
under a vacuum to remove the volatilized contaminant from within the pore
space of the soil. In some cases, removal of the contaminated air is passive,
meaning that the air is removed without the use of vacuum pressure through
a perforated pipe inserted into the contaminated area. However, active removal
is more efficient because of the increased pressure gradient (Chambers et al.
1991). The contaminated air that has been extracted must then be treated to
prevent air pollution problems. The gas stream is normally treated using an
adsorption method—for example, the use of activated carbon for lower con-
centrations and thermal oxidation for higher concentrations of the contaminant
(U.S. EPA 2006).
The volatility of the contaminant, the permeability of the soil (Stamnes and
Blanchard 1997), and site conditions are all significant factors in the success of
remediation using the soil vapor extraction process. Contaminant characteristics
that are important to consider for volatilization are the contaminant concentra-
tion, Henry's law constant, the carbon partition coefficient, vapor pressure, water
solubility, melting point, and the boiling point (LaGrega et al. 2001; Wong 1997).
Soil vapor extraction is found to be most successful for volatile organic com-
pounds, with a Henry's constant of 0.01 or a vapor pressure of 66 Pa or greater
(DePaoli et al. 1996). Soil properties of specific sites must be taken into consider-
ation, because it is essential for the flux of air to move through the contaminated
soil. For this reason, the air permeability and moisture content of the soil must
also be considered (Wong 1997).
Once soil vapor extraction has been selected as the method of treatment, it is
necessary to design a system that will be efficient in cleaning the site and that
will be reasonable in cost and duration. Modeling and pilot studies are important
in the process of designing a SVE system specific to a contaminated site that
will remove contaminants effectively.
SVE is a successful method for the remediation of petroleum-contaminated
soils. However, the process is limited to contaminated soils that are unsaturated
and permeable. Examples of suitable soils are gravels, sands, and coarse silts.
The system is very flexible in design, which makes it a more useful method of
remediation because it can operate specifically for a certain site. These parameters
include the spacing of the extraction wells, rate of air injected into the soil, and
variations in pumping. More wells can be added if necessary, but more wells
added to the system will increase the cost. Also, the more time the injected
air spends in the contaminated soil, the more efficient the treatment process
will be because it will allow plenty of time for the contaminant to vaporize.
Higher amounts of the contaminant can also be removed by increasing venting
(Chambers et al. 1991).